1. Field of the Invention
The present invention relates to a circuit for detecting a rotational position of a rotor and more specifically, for controlling the driving of a brushless direct-current (DC) motor. The present invention further relates to an energization timing determination circuit including the detecting circuit and to a determination method for the energization timing of a motor.
2. Description of Related Art
To control the driving of a brushless DC motor, multiple Hall elements or similar sensor elements are typically disposed at intervals of an angle of 60° or 120° on the stator side to detect the rotational position by way of the magnetic pole position of the rotor. Energization timing for stator coils is determined based on position signals from these elements. However, disadvantages arise in that when such a construction is adopted, a wiring for supplying power to the Hall elements and a wiring for obtaining detection output are required complicating the structure of the motor. In addition, Hall elements cannot be used in applications involving a high-temperature environment.
To cope with the above described and other disadvantages, a so-called sensorless drive system may be adopted. As described in JP-A-2006-158022, in such a sensorless drive system, a phase voltage produced in a stator coil is detected when a rotor is rotated. Rotor position information acquired from the phase voltage without the use of a sensor such as a Hall element.
FIG. 10 illustrates the general configuration of a system applied to a device for driving, for example, a vehicle fan motor mounted in a vehicle. A brushless DC motor 1 is driven through an inverter unit 2. The inverter unit 2 is constructed by connecting, for example, six power metal oxide semiconductor field effect transistors (MOSFET) 3a to 3f in a three-phase bridge configuration. The output terminals of the respective phases of the inverter unit 2 are respectively connected to the stator coils 4U, 4V, 4W of the respective phases of the motor 1.
The inverter unit 2 is controlled by a control unit 5 constructed of a microcomputer or a logic circuit, and the gate of each field effect transistor (FET) 3 has a driving signal outputted thereto through a gate driver circuit 6. The rotor rotational position of the motor 1 is detected by comparators 8 and a position detection unit 9 through low-pass filters 7 each constructed of a capacitor C and a resistor R, and a resulting position signal is supplied to a control circuit 10.
The input terminals of the low-pass filters 7U, 7V, 7W are connected to the output terminals of the respective phases of the inverter unit 2. The comparators 8U, 8V, 8W compare output signals of the low-pass filters 7U, 7V, 7W with a virtual neutral potential. The above construction excluding the motor 1 forms a motor driving device 11.
FIG. 11 illustrates voltage waveform at each part observed when the motor 1 is energized through the inverter unit 2. When the motor 1 is started, the control unit 5 energizes it in a predetermined pattern to start the motor 1. When the motor 1 starts to rotate, the induced voltages produced in the stator coils 4U, 4V, 4W appear as the terminal voltages of the coils 4 as shown in section (a. The terminal voltages of the coils 4 contain a harmonic component and a DC component. When the harmonic and DC components are removed through the low-pass filters 7, a substantially sinusoidal induced voltage waveform is obtained as shown in section (b. Then, the comparators 8 compare the output signals of the filters 7 with a virtual neutral potential and output rectangular-wave position signals of the respective phases as shown in section (c.
The control unit 5 sets a pulse width modulation (PWM) duty determining a rotational speed of the motor 1 according to a control signal supplied from an external electronic control unit (ECU), which for simplicity is not shown. At the same time, the control unit 5 determines commutation timing according to a position signal supplied from the position detection unit 9, and generates and outputs a driving signal to the gate driver circuit 6.
When a motor 1 is PWM controlled by a sensorless drive system, as mentioned above, an unwanted waveform component and noise contained in an induced phase voltage signal must be removed through a filter 7. As a result, a delay occurs in the phases of the induced voltage signal that passes through the filter 7. To make the phase delay substantially 90 degrees throughout the entire frequency range of the passed signal, a capacitance resistance (CR) time constant is increased to the extent that the attenuation of signals due to the filter 7 is permissible and a cut-off frequency is reduced.
It should be noted that since devices for driving a vehicle fan motor are typically installed in an engine compartment, the operating environmental temperature ranges vary widely. Further, due to tolerances of filters 7, CR time constants are susceptible to temperature resulting in considerable variation. Therefore, a high possibility exists that the time constant of each filter 7 of the respective phase will be shifted.
If there is an offset in a comparator 8 for comparing a signal that passes through a filter 7 with a virtual neutral potential, a discrepancy will be produced in position detection. A discrepancy is produced in the duty of a position signal by an offset. If there is variation in offset between three comparators 8U, 8V, 8W, the duty varies between three phases. If there is a discrepancy in position detection, energization timing varies and abnormal noise or degradation in efficiency due to torque ripples results.
The above problem pertains not only to sensorless drive systems. Also when a position sensor is used, energization timing will similarly vary with variation in the mounting location of each sensor.